1. Backstreet Rookie: (2020) A somewhat controversial choice (the depiction of the Rastafari character definitely has some unfortunate aspects to it), but the chemistry between the leads is fabulous (they even have their own sign language), and it's probably the funniest K-drama I've seen. It's really too bad a lot of people didn't give it a chance because of.the controversies. As far as the age difference between the leads, I really think that should be a non-issue. It has always been the case with Kim Yoo Jung that she is paired up with older actors, as her acting ability is so far above any of the males her age. In real life there is a 12 year difference between the leads, but in the show it's 7 years and they are both adults. It shouldn't be a big deal. Anyway, I strongly recommend this show. One of my favorites.
2. Mad For Each Other: (2021) A shorter one, (only 13 episodes at 30 minutes each) but another with great chemistry between the leads. They both have psychological hangups, and you find yourself really wanting these two to make each other happy. It's an enemies to frenemies to lovers story line, and done very well.
3. Marriage Not Dating: (2014) An older drama with, once again, terrific chemistry between the leads. (You can probably guess what I find essential. A rom/com without good chemistry is just a waste of everyone's time). The setup in this one is that for family reasons the ML has to pretend to have a fiance, so he makes a business deal with the FL, but of course their emotions take over eventually. This rom/com probably has more passion than just about any K-drama I've watched, with some pretty risque scenes. One scene in particular , , , Wow. The FL is the splendid Han Groo, who unfortunately (for us) gave birth to twins and retired from acting. There are also some wonderful secondary characters. All in all a great show.